The invention relates to a copy-protected optical disk of the type including at least one continuous spiral, and a copy-protection process to determine whether such a disk is an original or an illegal copy. The invention is notably applicable to CD-ROM and DVD-ROM type disks.
It is well known, notably in the software field, that developers of products suffer considerable financial loss due to illicit copying (also known as xe2x80x9cpiracyxe2x80x9d). This situation has recently been further aggravated by the availability on the mass consumer market of optical disk recorders that can write CD-R and CD-RW type disks that are very inexpensive to buy. Such devices can make perfect copies of the original CD-ROMs.
Numerous techniques have therefore been developed, in particular in recent years, to prevent illegal copying of optical disks. One of the simplest of these consist in writing a copy protection code at a pre-determined location on the disk during its fabrication. This pre-determined location is such that many copying techniques cannot reproduce this part of the disk. Readers are made to reject disks that do not have the protection code in the right location. However, it is obvious that a device designed or adapted to read all data at all locations on the disk can copy the disk including its protection code; the illegal copy obtained is then strictly identical to the original.
Another known technique is the SCMS (xe2x80x9cSerial Copy Management Systemxe2x80x9d) method according to which a disk carries a SCMS code that authorizes or forbids copying. A disk with a SCMS code authorizing copying can be copied, but the copying device changes the SCMS code so that further copies are not possible. It is clear that this technique suffers the same disadvantage as previously when the entire data content of the disk is copied.
Other relatively sophisticated techniques have been imagined to overcome the problem of unauthorized copying. Most of them involve the use of a specific xe2x80x9csignaturexe2x80x9d on the disk, which can consist in a variation of certain recording parameters, such as the form of the marks (depth, width, length), introduction of an asymmetry in the marks, wobbulation of the track at special frequencies, and so on. These variations constitute the signature that must be sought by the reader, and they cannot be reproduced by ordinary CD writers such as CD-R recorders. However, it is necessary that the disk reader be able to detect these variations, which is not generally the case with standard readers. A variant of this method enables creation of ambiguous code words that can be read with different values at successive reads of the disk with standard readers.
A different technique consists in deliberately destroying or damaging original disk""s spiral turns or sectors whose addresses can be encrypted to constitute a disk identifier code. However, one disadvantage of this type of technique is that it necessitates authentication of the disk""s user by means of more-or-less complex access information that the user must enter as a password to gain access to the content of the disk, via an authentication terminal. This technique therefore imposes non-negligible constraints. Another disadvantage of such methods of recognition of damaged parts is that they are able to hide only a small quantity of data, which makes it easier to incorporated them in the body of the software. Another problem is that the writing of such marks is structurally within the capabilities of commercially-available CD recorders, the only obstacle to recopying of disks being that the software controlling these CD recorders is unable to manage such marks, errors or omissions. A modification of the control software (either the user processor or the internal software of the CD recorder) would however be sufficient to copy these disks. We note here that the damaging of the disk can be simply limited to the omission of certain sectors.
To attempt to overcome some of these drawbacks and strengthen the security of anti-piracy systems making use of hidden codes, other techniques have been developed involving an interrupted spiral or a spiral with separated zones between which the data are written in such a manner as to prevent continuous recording of executable data. Such techniques may however imply reduced data density on the disk, or may require the use of non-standard readers.
One more promising avenue of development has been envisaged involving a disk with a continuous spiral or main track between whose turns is inserted a section of a secondary spiral, the standard pitch (separation) of the conventional optical disk tracks being conserved. An authentication method then consists in xe2x80x9crecognizingxe2x80x9d the secondary spiral only by verifying the presence of identification codes or specific addresses that are not found on the main track. However this technique does not apply efficiently the major advantage of having a zone that is not easily reproducible by a standard CD recorder.
The present invention overcomes these disadvantages and makes full use of the advantage of a such zone that prevents straightforward copying by standard CD recorders, thanks to recognition of the physical presence of a protection zone in two parts.
The invention is therefore a copy-protected optical disk of the type including at least one spiral track on which information marks are written in sectors whose addresses are substantially sequential along the track, wherein said disk includes:
a copy protection zone comprising two parts of equal size each including a series of sectors designated by identical addresses on each of the two parts, the data written in this zone being shared between the sectors of the two parts and each sector of the zone also carrying an identifier characteristic of the part to which the sector belongs; and
at least specific protection data to enable verification of the presence and constitution of said protection zone and to enable use of the data written in this zone, at least some elements of said specific protection data being recorded on said disk.
The advantage of the existence of a two-part protection zone is particularly important if one of the parts is written on a secondary track inserted between turns of the main track, since standard CD recorders are unable to write such a two-track disk, and it can be made very difficult to make any illegal copy of a disk by verifying the physical structure of the disk read. Furthermore, even a modification of the control electronics of an industrial device used to make master disks would not enable duplication of disks protected according to the invention.
The invention also includes an optical disk such as defined above, characterized in that it includes a main continuous spiral track covering all the useful part of the disk and whose sectors have addresses ordered substantially sequentially along this track, and at least one secondary track inserted between turns of said main track, the sectors on this secondary track and the adjacent sectors in a given radial direction on said main track having the same addresses, in order to constitute said two parts of said protection zone.
The invention also includes a copy-protection process to determine whether an optical disk is an original disk or an illegal copy and prevent normal use in the case of a copy, said original disk including at least one spiral track on which information marks are written in sectors whose addresses are substantially sequential along the track, said process being characterized in that it consists in:
defining on the original disk a protection zone in two parts of equal size each including a series of sectors designated by identical addresses on each of the two parts, the data written in this zone being shared between the sectors of the two parts and each sector of the zone also carrying an identifier (D1, D2) characteristic of the part to which the sector belongs;
performing a succession of reads of at least one predetermined sector of said protection zone;
verifying, by means of said identifiers, that during these reads, access has been made to the two predetermined sectors having the same address in each of two said parts of said protection zone.